Touch sensitive panels and screens are commonly used in many types of computerized equipment. In some systems, a touch screen avoids the need for providing a keyboard.
Referring to FIG. 1, a touch screen is typically used to allow the user to easily select one of a plurality of displayed items. The user makes his or her selection by touching the portion of the screen associated with the item to be selected. To clarify the boundaries of the areas associated with each item, the image on the screen may include boxes surrounding the displayed items.
The uses of touch screens have grown increasingly sophisticated, allowing the user to draw pictures, manipulate menus, use a displayed keyboard for alphanumeric input, and so on.
The terms "screen", "display", and "panel" are used synonymously herein. The present invention concerns the touch aspect of touch screens. Therefore, for this purpose it is unimportant how the image on the touch sensitive apparatus is displayed. The touch mechanism could even be used with a static image instead of with a display device.
In most cases, the term "touch sensitive" is a misnomer. Most touch sensitive screens, including the present invention, sense the interruption of one or more light beams; they do not sense actual physical contact with the screen or panel. The display is surrounded by pairs of light emitting and light sensing elements. These pairs are individually enabled in a preselected pattern, and the position of any object (such as the user's finger) touching the screen is determined by looking at which lights are blocked by the object.
A serious shortcoming in prior art touch screens is that their performance degrades in bright ambient light conditions, especially in sunlight. The source of this problem is as follows. In the prior art touch screens, the light detection system determines that the light traveling between a selected pair of emitting and receiving elements is blocked if the amplitude of the received light is below a threshold value. However, if the ambient light by itself causes the amount of light received by the screen's receiving elements to exceed the threshold value, then the system is unable to detect the presence of an object touching the screen.
In the more sophisticated prior art touch screens, the system compares the signal level output by the light receiving elements in response to ambient light with the signal level output when the light of a selected light emitting element is added to the ambient light. For instance, in U.S. Pat. No. 4,243,879, the disclosed system samples the signal level generated by each receiving element in response to the ambient light just before it turns on the corresponding light emitting element and compares the resulting signal level with sampled level.
This type of "calibrated threshold" prior art system suffers from very poor signal to noise ratios. In bright ambient light conditions the signal level attributable to the light emitting element will be very small, and therefore the incremental threshold for determining that light from the light emitting element is not blocked must be very low. However, the lower the threshold, the greater the chance that small ambient light fluctuations will drown out the signal from the light emitting element.
If a "calibrated threshold" system uses an ADC (analog to digital converter) to quantify the intensity of the light being received, when the system is in very bright ambient light (e.g., direct sunlight) the quantified ambient light level will be so large that the ADC will not be able to distinguish between ambient light and the light from unblocked light beams.
Also, the intensity of the light emitted by LEDs typically varies, from component to component, by a factor of up to ten to one. Also, the sensitivity of light receiving elements, which are usually phototransistors, vary even more than ten to one. In other words, the signal level generated by any two supposedly identical phototransistors, in response to the same light intensity level, can vary be even more than ten to one. While the problem of nonuniform components can be at least partially solved by sorting components, these variations generally force the prior art touch screens to use a fairly low incremental threshold for detecting unblocked light beams--which decreases the signal to noise ratio of those system. As a result, these touch screens often malfunction in bright ambient light conditions.
After studying these problems and the prior art solutions, the inventor of the present invention concluded that the use of a threshold intensity level is inherently problematic. Therefore the present invention uses a different concept.
In particular, the present invention modulates the light transmitted by the touch screen's LEDs, and then detects whether the light received by the screen's phototransistors includes a signal component that is modulated in the same way. If so, the light path is unblocked, otherwise the system concludes that light path is blocked. The inventor has found that this system works in all ambient light conditions, including bright, direct sunlight.
It is therefore a primary object of the present invention to provide a touch screen apparatus using modulated light that is operable even in strong ambient light conditions.